1. Field of the Invention
The present disclosure relates to a mercury-free arc tube for a discharge lamp device and a method for manufacturing the same, and more specifically, to a mercury-free arc tube in which regions including molybdenum foils are pinch-sealed by glass, and a method for manufacturing the same.
2. Description of Related Art
A related art discharge lamp device using an arc tube as a light source is structured so that front and rear end portions of the arc tube are supported on and integrated with an insulating base by a lead support which also serves as a conducting path protruding forward of the insulating base and a metallic holding member fixed to the front face of the insulating base.
The related art arc tube is structured so that, between a pair of front and rear pinch-sealed portions, a closed glass bulb in which electrodes are provided oppositely and luminescent substances, etc., are enclosed is formed. Inside the pinch-sealed portion, a molybdenum foil which connects the electrode projecting inside the closed glass bulb and a lead wire led out from the pinch-sealed portion is sealed to achieve airtightness of the pinch-sealed portion.
In other words, as an electrode, an electrode made of tungsten with excellent durability is advantageous. However, tungsten has a different linear expansion coefficient from glass and is not very compatible with glass, so that the airtightness of the related art arc tube is poor. Therefore, by connecting a molybdenum foil which is more satisfactorily compatible with glass, to the tungsten-made electrode and sealing the molybdenum foil at the pinch-sealed portion, airtightness of the pinch-sealed portion is secured.
Japanese Patent Unexamined Publication No. JP-A-2003-86136 proposes a related art technique in which, by roughening the surfaces of the molybdenum foils (forming minute unevenness) by applying etching including oxidation and reduction to the molybdenum foils sealed at the pinch-sealed portions of the arc tube for a discharge lamp device, physical adhesion (mechanical joining strength) to glass is improved, and foil floating is suppressed and the life of the arc tube becomes longer.
Japanese Patent Unexamined Publication No. JP-A-2002-33079 proposes a related art technique in which, in a lamp such as a halogen lamp including a glass-made bulb, a molybdenum foil connecting an electrode filament and an external conductor is sealed by glass. By coating discontinuous land regions of TiO2 on the surface of the molybdenum foil, a chemical joining force between TiO2 and glass is improved, and accordingly, the joining strength between the molybdenum foil and glass is improved, and the life of the lamp becomes longer.
The related art arc tubes described above are mercury-contained arc tubes in which mercury as a buffer substance is enclosed in a closed glass bulb. However, mercury is an environmental toxin, so that recently, a mercury-free arc tube in which mercury is not enclosed has attracted attention.